1. Field of the Invention
This invention relates to a multi-beam photoelectric sensor used for monitoring a person intruding into a hazardous area, or in particular to a multi-beam photoelectric sensor in which the length of the light projection and receiving columnar members, the number and the pitches of the optical axes can be changed in versatile fashion in accordance with the width of the hazardous area to be monitored or the minimum diameter of an object detected.
2. Description of the Related Art
The multi-beam photoelectric sensor of this type, as well known, comprises at least a light projection columnar member having a columnar case for accommodating a light projection string and at least a light-receiving columnar member having a columnar case for accommodating a photo-detector string, wherein the light projection and light-receiving columnar members are arranged with the light projection and light-receiving surfaces thereof in appropriately spaced and opposed relation to each other thereby to form an object-detecting light curtain between the light projection and light-receiving columnar members.
In the prior art, the light projector string and the photo-detector string accommodated in the columnar case is generally configured of a combination of the multi-beam optical modules having a unit number of optical axes (say, 4, 8 or 16 optical axes). In this multi-beam optical module, as many optical elements as the unit number of the optical axes (the light-emitting elements and the light projection lenses for the light projector, and the light-receiving elements and the light-receiving lenses for the photo-detector) are held integrally by a resin holder having a fixed pitch of optical axes.
In this conventional multi-beam photoelectric sensor, however, the length of the light projection and light-receiving columnar members and the number and the pitches of the optical axes are determined by a combination of the multi-beam optical modules having the fixed number and the fixed pitches of optical axes. It is therefore not easy to change the length of the light projection/light-receiving columnar member or the number and the pitches of optical axes in versatile way in accordance with the width of the hazardous area or the minimum diameter of the object to be detected.
In view of this, the present applicant has earlier proposed a novel multi-beam photoelectric sensor in which the light projector string and the photo-detector string accommodated in the columnar case are configured of a mass of single-beam optical modules (see Japanese Unexamined Patent Publication No. 2002-124170).
This novel multi-beam photoelectric sensor, in which the light projector string and the photo-detector string accommodated in the columnar case are configured of a mass of single-beam optical modules, can be fabricated by selecting the length of the light projection/light-receiving columnar member for each unit of optical axis. Thus, a light curtain most suitable for the width of a hazardous area to be monitored can be advantageously generated.
In the aforementioned multi-beam photoelectric sensor proposed earlier by the present applicant, the optical parts making up the projection optical system or the light-receiving optical system are accommodated in each module, and as compared with the other conventional light curtains, a light curtain having the number of optical axes most suitable for the width of the hazardous area to be monitored can be easily generated. With regard to the electrical parts such as the light projection circuit and the light-receiving circuit corresponding to each module, however, the circuit boards corresponding to a plurality of different optical modules are prepared and combined to meet the requirement of an arbitrary number of optical modules. Thus, it is necessary to hold a plurality of types of circuit boards in stock. As far as the circuit boards are concerned, therefore, the number of the parts of the circuit board in stock cannot be reduced to less than a predetermined amount.